The use of hydrogen as a source of energy is becoming desirable as emissions from industry, traffic and homes continue to pollute the environment. Using hydrogen as an alternative fuel for many different systems is advantageous because hydrogen does not pollute the environment and because it has the highest energy density per unit weight of any chemical fuel. When used as fuel in combustion engines or fuel cells, hydrogen is oxidized to water. Unlike other fuels, burning hydrogen does not produce carbon dioxide, which has been tied to global warming. Hydrogen has potential as a fuel in both mobile environments, such as vehicles, and stationary environments, such as utilities. Due to the advantages of using hydrogen as fuel, there exists a need to achieve higher net energy storage densities than are presently possible. For example, in mobile applications a net energy density comparable to that of gasoline, 2960 watt-hour/kg assuming an energy efficiency of 23%, may be necessary for hydrogen to be an effective fuel substitute for gasoline. No alternative method or device for storing hydrogen currently exists to achieve this value.
Several methods of storing hydrogen exist. In one method, hydrogen is compressed and stored as a gas under pressures of about 20 MPa or more. Limitations of this method include undesirable system weight-to-volume storage ratios due to the heavy walled containers needed to store the gas at high pressure.
Similarly, liquefaction of hydrogen and storage at cryogenic temperatures possess limitations. For example, significant energy penalties exist because of the high energy required to liquify the hydrogen and to maintain it in the liquified state. Although another method, cryogenic storage of hydrogen in high surface area activated carbons, avoids the energy costs of liquefying the hydrogen, other limitations exist such as a low storage capacity per kilogram of storage medium.
Metal hydride storage is another option for hydrogen storage. In this system, a metal such as magnesium, vanadium, titanium or niobium reversibly forms a metal hydride by absorbing hydrogen in an exothermic reaction. Upon application of heat, the hydride disassociates into the metal and hydrogen, thus allowing the hydrogen to be used as fuel. Magnesium hydride is the preferred hydride because of its high weight percentage of hydrogen, 7.6%. Pure magnesium hydride, however, has poor hydriding and dehydriding kinetics. For example, it must be heated to about 300 C. to release hydrogen. This temperature cannot be readily achieved using waste heat from a combustion engine. Consequently, researchers have examined alternative magnesium compounds such as Mg.sub.2 NiH.sub.x, La.sub.2 Mg.sub.17 H.sub.x and Mg.sub.2 CuH.sub.x as well as non-magnesium compounds such as FeTiH.sub.x, LaNi.sub.5 H.sub.x and CaNi.sub.5 H.sub.x. Although these alternatives have better hydriding and dehydriding kinetics, none of them contains a weight percentage of hydrogen that exceeds magnesium hydride.
The present metal hydrides also possess charge/discharge cycle limitations, which are a consequence of the hydride material decrepitating and compacting within its containment vessel after several charge/discharge cycles. Designing systems to compensate for compaction results in a loss of hydrogen storage per unit weight. Thus, the existing metal hydride systems are heavy and typical system energy storage densities are about 1/16th or less of that for gasoline. The maximum value for metal hydride storage, assuming 7.6 weight percent (wt. % ) of hydrogen, is about 700 watt-hours/kg for an automotive engine efficiency of about 30%.
Physical storage of hydrogen using materials such as charcoal, zeolite or glass powder is another option. The resulting net energy densities in such systems, however, may also be insufficient for mobile applications. For example, the net energy density of hydrogen stored in charcoal at room temperature is only about 10 watt-hours/kg, which is substantially less than gasoline. Thus, the prior art currently has no practical way to enable hydrogen to be an effective fuel substitute for gasoline because the achievable net energy densities are not high enough.
Accordingly, there is a need for a hydrogen storage device capable of achieving higher net energy densities than are presently possible.